I. Field of the Invention
This invention generally pertains to the art of liquid level sensing devices, and more particularly to low temperature applications (such as storage tanks for fuels and the like) wherein conventional liquid level sensing devices must be continually connected to operating power sources in order to operate properly.
II. Description of the Prior Art
U.S. Pat. Nos. 3,340,527 (Rowell) and 3,461,446 (Sargeant) disclose the use of negative temperatures coefficient thermistors (hereinafter called NTC thermistors) in liquid level sensing device. However, these devices cannot successfully operate after they have remained in very cold environments with the power disconnected. This invention is designed to permit liquid level sensing under these conditions, and is therefore distinguishable over the prior art.
III. Summary of the Invention
A conventional liquid level sensing system has a power source, a standard resistance, and an NTC thermistor all connected in series, with a voltmeter connected across the resistance. Typically, the power source is on the order of 12 V DC. Since an NTC thermistor has a resistance that decreases with its own temperature, the current through the resistance (and thus the voltage across it) increases with increasing temperature.
In conventional practice, the NTC thermistor is encapsulated and placed in a liquid containing tank, and the power source is turned on. As current flows through the NTC thermistor, it heats up and its resistance decreases until a steady state condition is reached, in which both the temperature of the NTC thermistor and the current flowing through it remain constant. When the encapsulated thermistor (hereinafter called the probe) is located in that portion of the tank which does not contain liquid, heat dissipation from the probe is minimized, and the probe temperature stabilizes at a high value. However, when the liquid in the tank reaches the level of the probe and partially or totally immerses it, heat dissipation is maximized and the probe temperature stabilized at a lower value.
It can thus be seen by one skilled in the art that be reading the voltmeter, the level of the liquid relative to the probe in question can be determined, since a stable high voltage reading will indicate that the liquid in the tank has not reached the level of the probe, and a stable low voltage reading will indicate that the probe has been at least partially immersed by the liquid in the tank. By using a vertically aligned, equally spaced-apart arrangement of probes, the level of the liquid in the tank can be determined to an accuracy equal to the distance between any two adjacent probes.
Although all this is known in the art, it will appear to one skilled therein that in order for the NTC thermistor to be used in this fashion, it must be heated to such a range (which range is herinafter referred to as its operating temperature range) that a steady state condition can be established at a non-zero current flow value through the thermistor. If the NTC thermistor is too cold, its resistance will be so high (approximately 1/2 megohm) that, at conventional source voltages, it will act like an open circuit, and will never heat up enough to be usable in the fashion above described. Thus, if a conventional probe is used in a cold environment, e.g. -40.degree. F., it must be continuously connected to an operating power source. Otherwise, it will cool down too much and will not be usable.
In this invention, a heater is provided which works on the same power supply as is used with the NTC thermistor. As will appear hereinafter, this heater is used to raise the temperature of the NTC thermistor to a point at which the NTC thermistor will begin to draw sufficient power to heat itself up and eventually reach its operating temperature range. Moreover, this heater is so designed as not to interfere with the measuring function of the NTC thermistor after it has reached its operating temperature range.